DarkSkies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the DarkSkies Observatory clear sky chart from 2005-11-06 to 2024-02-20.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.
Observation TypeMinimum conditions for hour to be usableOut of hours
Clear of CloudsTransparencySeeingSky Brightness
Planetary:50%(no limit)Average(no limit)Nighttime Hours
Deep Sky:70%Average(no limit)5.9Nighttime Hours
CCD Imaging:70%Below AveragePoor(no limit)Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

PlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
Night
22%
23045
18%
80421
6%
22950
36%
22950
17%
52053

By Month

MonthDays of MonthPlanetaryData
Pts
Deep Sky
Max 60%
Data
Pts
Both Planetary and Deep Sky
Max 60%
Data
Pts
Either Planetary or Deep SkyData
Pts
CCD ImagingData
Pts
January1st to 10thNight
9%
753
22%
4329
3%
737
29%
737
9%
2353
January11th to 20thNight
11%
758
20%
4348
3%
740
27%
740
7%
2635
January21st to endNight
12%
831
12%
4861
4%
814
31%
814
7%
3449
February1st to 10thNight
11%
760
17%
2421
2%
760
28%
760
14%
1615
February11th to 20thNight
9%
742
19%
2215
2%
739
28%
739
14%
1462
February21st to endNight
10%
592
22%
1776
5%
592
32%
592
13%
1190
March1st to 10thNight
13%
720
21%
2160
6%
720
33%
720
17%
1414
March11th to 20thNight
14%
720
17%
2091
4%
720
27%
720
14%
1450
March21st to endNight
18%
792
21%
2016
4%
792
31%
792
18%
1403
April1st to 10thNight
16%
720
19%
1800
3%
720
26%
720
18%
1277
April11th to 20thNight
20%
711
16%
1775
2%
711
28%
711
21%
1242
April21st to endNight
17%
554
19%
1634
3%
554
29%
554
17%
1116
May1st to 10thNight
18%
540
13%
1620
4%
540
26%
540
15%
1166
May11th to 20thNight
19%
457
16%
1519
6%
457
30%
457
17%
1006
May21st to endNight
29%
389
13%
1551
9%
388
39%
388
17%
1004
June1st to 10thNight
26%
360
14%
1440
9%
360
41%
360
15%
953
June11th to 20thNight
31%
360
12%
1440
11%
360
40%
360
16%
943
June21st to endNight
32%
360
13%
1440
10%
360
43%
360
21%
879
July1st to 10thNight
41%
350
13%
1400
14%
350
49%
350
21%
869
July11th to 20thNight
43%
348
12%
1392
13%
348
49%
348
21%
851
July21st to endNight
46%
394
18%
1486
17%
372
54%
372
25%
846
August1st to 10thNight
43%
444
17%
1508
10%
442
50%
442
25%
910
August11th to 20thNight
40%
594
15%
1658
8%
594
43%
594
24%
1119
August21st to endNight
45%
792
21%
1980
10%
792
50%
792
34%
1284
September1st to 10thNight
39%
720
20%
1800
7%
720
46%
720
31%
1188
September11th to 20thNight
39%
720
25%
1956
12%
720
48%
720
33%
1199
September21st to endNight
35%
720
24%
1980
14%
720
45%
720
28%
1271
October1st to 10thNight
29%
720
17%
1980
9%
720
38%
720
22%
1364
October11th to 20thNight
28%
720
21%
2214
8%
720
44%
720
24%
1385
October21st to endNight
20%
792
15%
2574
6%
792
32%
792
16%
1755
November1st to 10thNight
24%
740
22%
2407
9%
740
40%
740
22%
1486
November11th to 20thNight
19%
760
23%
2470
8%
760
38%
760
17%
1546
November21st to endNight
15%
760
22%
2489
5%
760
34%
760
17%
1556
December1st to 10thNight
13%
760
17%
2660
4%
760
29%
760
13%
1739
December11th to 20thNight
14%
760
16%
3222
3%
759
30%
759
11%
2015
December21st to endNight
12%
832
14%
4809
3%
817
31%
817
7%
3113

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

Occurrence of each ForecastData
Points
Cloud CoverDay
16%7%36%5%35%5%5%7%38%
78226
Night
16%8%36%5%3445%7%39%
80949
TransparencyDay
44%7%12%21%14%2
77931
Night
43%7%12%22%14%2
80778
SeeingDay
23%19%32%18%7%2
13956
Night
22%18%34%21%5%
23063

By Month

Percentage Cloud Cover NE (2006-)
Color Meanings:
Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
13%9%37%5%35%5%5%6%41%
2517
11th to 20thDay
15%8%37%7%45%5%5%5%37%
2520
21st to endDay
19%13%46%5%35%5%45%32%
2772
February1st to 10thDay
17%7%36%5%5%6%5%5%7%34%
1753
11th to 20thDay
12%7%37%6%45%5%5%8%37%
1838
21st to endDay
18%10%37%6%35%44435%
1480
March1st to 10thDay
14%9%36%6%45%5%47%38%
1797
11th to 20thDay
16%8%36%6%36%45%7%36%
1867
21st to endDay
18%8%46%5%34446%39%
2335
April1st to 10thDay
16%9%37%6%34446%37%
2160
11th to 20thDay
15%7%36%5%35%5%7%7%37%
2134
21st to endDay
21%7%35%7%4445%6%33%
2326
May1st to 10thDay
24%8%35%6%345%5%7%31%
2340
11th to 20thDay
19%7%26%5%34446%40%
2390
21st to endDay
15%5%36%5%35%45%8%40%
2725
June1st to 10thDay
19%6%35%6%45%6%6%8%33%
2520
11th to 20thDay
18%6%35%6%35%5%6%9%35%
2520
21st to endDay
17%7%36%5%46%5%6%9%32%
2520
July1st to 10thDay
13%6%35%5%35%5%7%8%39%
2520
11th to 20thDay
12%5%26%5%46%6%7%9%38%
2520
21st to endDay
13%6%26%5%45%6%7%8%38%
2744
August1st to 10thDay
11%6%25%6%46%6%7%9%37%
2400
11th to 20thDay
13%6%26%6%45%5%6%9%37%
2239
21st to endDay
12%5%25%5%36%5%7%9%41%
2376
September1st to 10thDay
13%6%26%5%25%5%5%8%42%
2160
11th to 20thDay
15%7%2432345%5%50%
2003
21st to endDay
16%6%35%4335%5%7%44%
1980
October1st to 10thDay
23%7%35%5%3434438%
1980
11th to 20thDay
13%7%344346%5%7%44%
1742
21st to endDay
21%8%36%6%3345%6%37%
1782
November1st to 10thDay
14%7%36%323445%48%
1671
11th to 20thDay
19%7%24433446%45%
1710
21st to endDay
17%8%35%5%3445%6%41%
1687
December1st to 10thDay
17%8%47%5%36%5%46%35%
1520
11th to 20thDay
21%7%2443445%6%40%
1906
21st to endDay
20%11%46%6%35%45%7%30%
2772
January1st to 10thNight
16%8%35%42445%6%44%
4429
11th to 20thNight
13%9%38%7%35%447%37%
4440
21st to endNight
21%11%5%7%6%35%445%29%
4884
February1st to 10thNight
15%9%36%5%345%5%7%38%
2421
11th to 20thNight
12%8%45%6%35%6%6%7%39%
2213
21st to endNight
16%9%36%5%3445%7%38%
1776
March1st to 10thNight
14%8%36%5%3445%7%41%
2150
11th to 20thNight
15%7%36%6%46%5%5%7%36%
2091
21st to endNight
16%8%36%5%25%5%6%6%40%
2016
April1st to 10thNight
17%8%36%6%346%6%6%36%
1800
11th to 20thNight
18%6%35%5%3445%7%40%
1775
21st to endNight
21%6%37%5%3445%6%38%
1634
May1st to 10thNight
20%10%37%5%35%45%6%30%
1620
11th to 20thNight
18%7%36%5%345%5%7%37%
1518
21st to endNight
16%7%36%6%25%45%8%38%
1550
June1st to 10thNight
18%8%5%5%5%34446%37%
1440
11th to 20thNight
19%7%36%5%345%6%7%37%
1440
21st to endNight
14%5%25%4345%6%9%42%
1440
July1st to 10thNight
14%5%35%5%36%5%5%8%41%
1440
11th to 20thNight
14%5%26%6%35%5%6%11%38%
1440
21st to endNight
11%4245%45%5%6%8%45%
1574
August1st to 10thNight
9%5%26%5%36%5%6%8%44%
1518
11th to 20thNight
13%6%36%6%45%5%7%7%37%
1658
21st to endNight
14%6%1443336%9%47%
1980
September1st to 10thNight
13%6%345%3336%7%47%
1800
11th to 20thNight
13%6%25%5%23347%51%
1956
21st to endNight
14%5%345%34447%46%
1980
October1st to 10thNight
20%7%25%5%24445%40%
1980
11th to 20thNight
15%6%25%4345%6%7%43%
2214
21st to endNight
20%9%35%43435%7%37%
2574
November1st to 10thNight
15%6%25%425%45%6%47%
2407
11th to 20thNight
17%6%25%5%34346%44%
2470
21st to endNight
14%7%35%43446%7%42%
2489
December1st to 10thNight
18%9%35%5%345%5%7%36%
2660
11th to 20thNight
18%7%25%5%346%5%6%38%
3288
21st to endNight
19%11%46%43445%6%33%
4884

Transparency (when cloud cover <= 30%)
Color Meanings:
Too cloudy to forecast Poor Below Average Average Above average Transparent
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
41%5%11%32%9%2
2518
11th to 20thDay
47%6%12%23%10%2
2520
21st to endDay
52%6%9%21%10%2
2772
February1st to 10thDay
49%6%13%19%11%2
1753
11th to 20thDay
44%8%10%20%15%3
1847
21st to endDay
51%6%8%18%14%3
1480
March1st to 10thDay
45%5%13%20%13%4
1800
11th to 20thDay
46%5%12%18%14%4
1867
21st to endDay
47%5%8%17%17%5%
2335
April1st to 10thDay
48%5%9%19%15%3
2160
11th to 20thDay
43%7%12%19%17%3
2134
21st to endDay
50%6%8%17%16%3
2326
May1st to 10thDay
53%5%10%19%12%2
2340
11th to 20thDay
45%6%9%21%17%2
2394
21st to endDay
41%8%13%23%13%2
2725
June1st to 10thDay
46%8%15%21%9%1
2520
11th to 20thDay
44%8%13%19%14%2
2520
21st to endDay
47%8%13%21%11%1
2520
July1st to 10thDay
41%7%13%22%15%2
2450
11th to 20thDay
39%9%14%25%11%1
2445
21st to endDay
39%8%14%22%14%3
2591
August1st to 10thDay
40%9%15%23%12%1
2386
11th to 20thDay
42%8%15%23%11%1
2239
21st to endDay
36%8%12%26%16%3
2376
September1st to 10thDay
39%6%13%24%16%2
2160
11th to 20thDay
36%5%9%25%23%2
2003
21st to endDay
39%5%10%23%20%3
1980
October1st to 10thDay
49%6%9%20%13%4
1980
11th to 20thDay
37%7%13%28%13%2
1742
21st to endDay
48%5%11%20%12%3
1782
November1st to 10thDay
39%6%12%25%16%3
1671
11th to 20thDay
41%5%11%21%19%3
1710
21st to endDay
42%7%15%23%11%2
1687
December1st to 10thDay
47%7%14%16%15%1
1520
11th to 20thDay
43%6%14%18%17%2
1906
21st to endDay
51%8%10%19%11%1
2772
January1st to 10thNight
39%6%13%28%11%3
4429
11th to 20thNight
46%6%13%21%11%3
4440
21st to endNight
56%6%10%16%10%2
4884
February1st to 10thNight
45%7%15%19%12%2
2421
11th to 20thNight
42%7%12%19%16%3
2215
21st to endNight
45%5%10%22%14%3
1776
March1st to 10thNight
43%5%11%24%14%3
2160
11th to 20thNight
46%7%12%16%16%4
2092
21st to endNight
44%6%10%17%18%5%
2016
April1st to 10thNight
46%6%11%19%15%3
1800
11th to 20thNight
43%7%11%20%18%1
1775
21st to endNight
47%5%10%19%14%5%
1634
May1st to 10thNight
52%6%10%18%11%3
1620
11th to 20thNight
45%7%13%18%15%3
1519
21st to endNight
44%9%12%21%13%1
1551
June1st to 10thNight
47%8%9%24%11%1
1440
11th to 20thNight
46%8%12%20%13%2
1440
21st to endNight
37%10%15%24%14%
1440
July1st to 10thNight
39%10%13%23%15%1
1400
11th to 20thNight
40%8%18%24%10%
1392
21st to endNight
34%8%17%27%12%2
1486
August1st to 10thNight
35%9%15%24%15%1
1508
11th to 20thNight
41%9%17%21%11%
1658
21st to endNight
34%7%12%27%18%1
1980
September1st to 10thNight
37%7%13%25%18%1
1800
11th to 20thNight
34%5%11%23%24%2
1956
21st to endNight
38%6%9%27%17%3
1980
October1st to 10thNight
45%6%11%23%12%3
1980
11th to 20thNight
38%7%14%27%13%1
2214
21st to endNight
48%6%13%20%12%2
2574
November1st to 10thNight
38%7%12%21%19%3
2407
11th to 20thNight
41%5%12%21%18%3
2470
21st to endNight
40%6%15%25%13%1
2489
December1st to 10thNight
45%7%14%21%12%1
2660
11th to 20thNight
43%7%17%17%15%2
3288
21st to endNight
49%6%11%23%10%1
4884

Seeing (when cloud cover <= 80%)
Color Meanings:
Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
MonthDays in MonthOccurrence of each ForecastData
Points
January1st to 10thDay
30%27%26%16%1
180
11th to 20thDay
31%27%31%8%3
180
21st to endDay
23%30%34%10%2
198
February1st to 10thDay
32%30%28%9%1
335
11th to 20thDay
30%31%31%8%
452
21st to endDay
33%23%33%10%1
346
March1st to 10thDay
29%25%37%7%2
420
11th to 20thDay
26%34%28%9%31
420
21st to endDay
30%23%32%9%5%1
462
April1st to 10thDay
32%25%28%9%5%
420
11th to 20thDay
26%24%34%11%41
412
21st to endDay
32%20%30%14%31
420
May1st to 10thDay
34%23%24%15%31
423
11th to 20thDay
20%20%40%15%32
447
21st to endDay
19%14%34%22%9%1
485
June1st to 10thDay
20%15%37%19%8%2
450
11th to 20thDay
18%15%39%20%7%1
450
21st to endDay
17%15%39%19%8%2
450
July1st to 10thDay
18%9%30%29%12%2
445
11th to 20thDay
15%8%33%31%10%2
444
21st to endDay
13%10%36%30%9%2
488
August1st to 10thDay
13%10%33%30%13%2
446
11th to 20thDay
17%8%33%26%14%2
439
21st to endDay
14%11%32%28%11%4
495
September1st to 10thDay
19%11%28%23%14%5%
450
11th to 20thDay
20%14%27%20%12%7%
450
21st to endDay
18%15%32%20%10%5%
450
October1st to 10thDay
30%13%26%18%11%3
450
11th to 20thDay
17%20%34%21%7%1
413
21st to endDay
30%17%31%17%5%1
396
November1st to 10thDay
21%21%30%18%8%1
365
11th to 20thDay
31%24%26%16%31
370
21st to endDay
29%21%31%14%5%
347
December1st to 10thDay
24%32%28%12%3
180
11th to 20thDay
37%24%32%5%1
180
21st to endDay
29%31%28%7%41
198
January1st to 10thNight
23%26%38%12%2
760
11th to 20thNight
27%25%31%14%2
760
21st to endNight
20%25%37%15%2
836
February1st to 10thNight
23%24%38%14%1
760
11th to 20thNight
23%28%37%12%1
742
21st to endNight
27%26%33%12%1
592
March1st to 10thNight
25%24%35%14%2
720
11th to 20thNight
23%26%34%14%2
720
21st to endNight
24%24%31%17%4
792
April1st to 10thNight
25%21%34%17%2
720
11th to 20thNight
23%22%32%18%5%
711
21st to endNight
25%20%35%18%2
554
May1st to 10thNight
27%16%35%19%2
540
11th to 20thNight
20%17%38%21%4
457
21st to endNight
20%10%36%30%5%
389
June1st to 10thNight
20%12%36%25%8%
360
11th to 20thNight
20%9%36%30%5%1
360
21st to endNight
16%443%32%5%
360
July1st to 10thNight
15%6%33%37%9%
350
11th to 20thNight
16%430%39%9%1
348
21st to endNight
10%5%36%39%9%
394
August1st to 10thNight
11%5%35%38%12%
444
11th to 20thNight
13%6%31%37%12%1
594
21st to endNight
17%5%27%36%14%1
792
September1st to 10thNight
16%7%33%32%10%1
720
11th to 20thNight
16%9%31%26%16%2
720
21st to endNight
18%11%32%26%12%2
720
October1st to 10thNight
26%13%28%23%9%2
720
11th to 20thNight
18%17%33%23%8%1
720
21st to endNight
27%18%31%19%5%
792
November1st to 10thNight
19%19%35%19%7%1
740
11th to 20thNight
23%24%30%16%6%1
760
21st to endNight
22%24%35%15%4
760
December1st to 10thNight
27%23%34%13%4
760
11th to 20thNight
26%24%33%14%2
760
21st to endNight
27%26%31%14%1
836
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